Terminal, base station, communication system, and communication method

ABSTRACT

There is provided a terminal for performing OFDM (Orthogonal Frequency-Division Multiplexing) communication with a base station forming a cell. The terminal includes a control channel processing unit configured to monitor, in a sub-frame, a first control channel in a cell-specific search space that is a search space common within the cell, and a first control channel in a UE-specific search space that is a search space specific to the terminal. In a case where monitoring of a second control channel different from the first control channel that is in the cell-specific search space or the UE-specific search space is configured, the control channel processing unit monitors, in a sub-frame, at least both the first control channel in the cell-specific search space and the second control channel in the UE-specific search space.

TECHNICAL FIELD

The present invention relates to a terminal, a base station, a communication system, and a communication method.

BACKGROUND ART

In wireless communication systems complying with the 3GPP (Third Generation Partnership Project) WCDMA (Wideband Code Division Multiple Access), LTE (Long Term Evolution), and LTE-A (LTE-Advanced) standards, the IEEE (The Institute of Electrical and Electronics engineers) Wireless LAN and WiMAX (Worldwide Interoperability for Microwave Access) standards, and so forth, a base station (cell, transmit station, transmitter, eNodeB) and a terminal (mobile terminal, receive station, mobile station, receiver, UE (User Equipment)) each include a plurality of transmit/receive antennas, and employ MIMO (Multi Input Multi Output) technology to spatially multiplex data signals to realize high-speed data communication.

In these wireless communication systems, in order to realize data communication between a base station and a terminal, it is necessary for the base station to perform various kinds of control for the terminal. To do this, the base station notifies the terminal of control information by using certain resources to perform data communication in the downlink and uplink. For example, the base station notifies the terminal of information on the allocation of resources, information on the modulation and coding of data signals, number-of-spatial-multiplexing-layers information on data signals, transmit power control information, and so forth to implement data signals. Transmission of such control information is realized using the method described in NPL 1.

Various methods can be used as communication methods based on MIMO technology in the downlink. For example, a multi-user MIMO scheme in which the same resources are allocated to different terminals, a CoMP (Cooperative Multipoint) scheme in which a plurality of base stations coordinate with each other to perform data communication, and so forth can be employed.

FIG. 14 is a diagram illustrating an example in which the multi-user MIMO scheme is implemented. In FIG. 14, a base station 1401 performs data communication with a terminal 1402 via a downlink 1404, and performs data communication with a terminal 1403 via a downlink 1405. In this case, the terminal 1402 and the terminal 1403 perform multi-user MIMO-based data communication. The downlink 1404 and the downlink 1405 use the same resources in the frequency direction and the time direction. Further, the downlink 1404 and the downlink 1405 each control beams using a precoding technique and so forth to mutually maintain orthogonality or reduce co-channel interference. Accordingly, the base station 1401 can realize data communication with the terminal 1402 and the terminal 1403 using the same resources.

FIG. 15 is a diagram illustrating an example in which the CoMP scheme is implemented. In FIG. 15, the establishment of a wireless communication system having a heterogeneous network configuration using a macro base station 1501 with a broad coverage and a RRH (Remote Radio Head) 1502 with a narrower coverage than this macro base station is illustrated. Now, consideration is given of the case where the coverage of the macro base station 1501 includes part or all of the coverage of the RRH 1502. In the example illustrated in FIG. 15, the macro base station 1501 and the RRH 1502 establish a heterogeneous network configuration, and coordinate with each other to perform data communication with a terminal 1504 via a downlink 1505 and a downlink 1506, respectively. The macro base station 1501 is connected to the RRH 1502 via a line 1503, and can transmit and receive a control signal and/or a data signal to and from the RRH 1502. The line 1503 may be implemented using a wired line such as a fiber optic line, a wireless line that is based on relay technology, or the like. In this case, the macro base station 1501 and the RRH 1502 use frequencies (resources) some or all of which are identical, thereby improving the total frequency utilization efficiency (transmission capacity) within a coverage area established by the macro base station 1501.

The terminal 1504 can perform single-cell communication with the macro base station 1501 or the RRH 1502 while located near the macro base station 1501 or the RRH 1502. While located near the edge (cell edge) of the coverage established by the RRH 1502, the terminal 1504 needs to take measures against co-channel interference from the macro base station 1501. There has been proposed a method for reducing or suppressing interference with the terminal 1504 in the cell-edge area by using the COMP scheme in which neighboring base stations coordinate with each other for multi-cell communication (cooperative communication) between the macro base station 1501 and the RRH 1502. As the CoMP scheme, for example, the method described in NPL 2 has been proposed.

CITATION LIST Non Patent Literature

-   NPL 1: 3rd Generation Partnership Project; Technical Specification     Group Radio Access Network; Evolved Universal Terrestrial Radio     Access (E-UTRA); Physical layer procedures (Release 10), March 2011,     3GPP TS 36.212 V10.1.0 (2011-03). -   NPL 2: 3rd Generation Partnership Project; Technical Specification     Group Radio Access Network; Further Advancements for E-UTRA Physical     Layer Aspects (Release 9), March 2010, 3GPP TR 36.814 V9.0.0     (2010-03).

SUMMARY OF INVENTION Technical Problem

In a wireless communication system capable of MIMO communication based on a scheme such as the multi-user MIMO scheme or the CoMP scheme, however, due to the improvement in transmission capacity achievable with one base station, the number of terminals that can be accommodated also increases. For this reason, in a case where a base station notifies terminals of control information using conventional resources, resources allocated to the control information may be insufficient. In this case, it is difficult for the base station to efficiently allocate data to terminals, which may hinder the improvement of transmission efficiency.

The present invention has been made in view of the foregoing problems, and it is an object of the present invention to provide a base station, a terminal, a communication system, and a communication method that allow the base station to efficiently notify the terminal of control information in a wireless communication system in which the base station and the terminal communicate with each other.

Solution to Problem

According to an embodiment, there is provided a terminal for performing OFDM (Orthogonal Frequency-Division Multiplexing) communication with a base station forming a cell. The terminal includes a control channel processing unit configured to monitor, in one sub-frame, a first control channel in a cell-specific search space that is a search space common within the cell, and a first control channel in a UE-specific search space that is a search space specific to the terminal. In a case where monitoring of a second control channel different from the first control channel in the cell-specific search space or the UE-specific search space is set, the control channel processing unit at least monitors, in the one sub-frame, both the first control channel in the cell-specific search space and the second control channel in the UE-specific search space.

Preferably, the first control channel in the cell-specific search space is a control channel which can include system information, paging, and control information concerning random access instruction.

Preferably, the first control channel in the cell-specific search space is transmitted using a transmission port used for transmission of a cell-specific reference signal, and the second control channel in the UE-specific search space is transmitted using a transmission port used for transmission of a UE-specific reference signal.

Preferably, the first control channel in the cell-specific search space is arranged over OFDM symbols up to a certain number of OFDM symbols from the top OFDM symbol in the sub-frame, and the second control channel in the UE-specific search space is arranged in OFDM symbols subsequent to the last one of the certain number of OFDM symbols in the sub-frame.

Preferably, the first control channel in the cell-specific search space is a control channel usable while the terminal is in idle state and in connected state, and the second control channel in the UE-specific search space is a control channel usable only while the terminal is in the connected state.

According to another embodiment, there is provided a base station that forms a cell and that performs OFDM communication with a terminal. The base station includes a transmission unit configured to notify the terminal of configuration information indicating that both a first control channel in a cell-specific search space that is a search space common within the cell and a second control channel in a UE-specific search space that is a search space specific to the terminal are at least monitored in one sub-frame.

Preferably, the transmission unit transmits the first control channel in the cell-specific search space using a transmission port used for transmission of a cell-specific reference signal, and transmits the second control channel in the UE-specific search space using a transmission port used for transmission of as a UE-specific reference signal.

Preferably, the transmission unit transmits the first control channel in the cell-specific search space via OFDM symbols up to a certain number of OFDM symbols from the top OFDM symbol in the sub-frame, and transmits the second control channel in the UE-specific search space via OFDM symbols subsequent to the last one of the certain number of OFDM symbols in the sub-frame.

Preferably, the first control channel in the cell-specific search space is a control channel usable while the terminal is in idle state and in connected state, and the second control channel in the UE-specific search space is a control channel usable only while the terminal is in the connected state.

According to still another embodiment, there is provided a communication system in which a base station forming a cell and a terminal perform OFDM communication. The base station includes a transmission unit configured to notify the terminal of configuration information on a second control channel. The terminal includes a control channel processing unit configured to monitor, in one sub-frame, a first control channel in a cell-specific search space that is a search space common within the cell, and a first control channel in a UE-specific search space that is a search space specific to the terminal. In a case where monitoring of the second control channel is set, the control channel processing unit at least monitors, in one sub-frame, both the first control channel in the cell-specific search space and the second control channel in the UE-specific search space.

According to still another embodiment, there is provided a communication method for a terminal that performs OFDM communication with a base station forming a cell. The communication method includes a step of monitoring, in one sub-frame, a first control channel in a cell-specific search space that is a search space common within the cell, and a first control channel in a UE-specific search space that is a search space specific to the terminal; and a step of, in a case where monitoring of a second control channel different from the first control channel in the cell-specific search space or the UE-specific search space is set, at least monitoring, in the one sub-frame, both the first control channel in the cell-specific search space and the second control channel in the UE-specific search space.

According to still another embodiment, there is provided a communication method for a base station that forms a cell and that performs OFDM communication with a terminal. The communication method includes a step of notifying the terminal of configuration information indicating that both a first control channel in a cell-specific search space that is a search space common within the cell, and a second control channel in a UE-specific search space that is a search space specific to the terminal are at least monitored in one sub-frame.

Advantageous Effects of Invention

According to this invention, in a wireless communication system in which a base station and a terminal communicate with each other, the base station can efficiently notify the terminal of control information.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating a communication system for performing data transmission according to a first embodiment of the present invention.

FIG. 2 is a schematic block diagram illustrating a configuration of a base station according to the first embodiment of the present invention.

FIG. 3 is a schematic block diagram illustrating a configuration of a terminal according to the first embodiment of the present invention.

FIG. 4 is a diagram illustrating an example of one resource block pair that the base station maps.

FIG. 5 is a diagram illustrating an example of channels that the base station maps.

FIG. 6 is a diagram illustrating the flow for setting UE-specific configuration information for radio resources.

FIG. 7 is a diagram illustrating an example of the UE-specific configuration information for radio resources.

FIG. 8 is a diagram illustrating an example of UE-specific configuration information on a second control channel.

FIG. 9 is a diagram illustrating the flow for a process for receiving a control channel and a data channel at the terminal.

FIG. 10 is a diagram illustrating an example of frequency arrangement for cells with carrier aggregation according to a second embodiment of the present invention.

FIG. 11 is a diagram illustrating another example of the UE-specific configuration information for radio resources.

FIG. 12 is a diagram illustrating another example of the UE-specific configuration information for radio resources.

FIG. 13 is a diagram illustrating another example of the UE-specific configuration information on the second control channel.

FIG. 14 is a diagram illustrating an example in which the multi-user MIMO scheme is implemented.

FIG. 15 is a diagram illustrating an example in which the CoMP scheme is implemented.

DESCRIPTION OF EMBODIMENTS First Embodiment

A first embodiment of the present invention will be described hereinafter. A communication system according to the first embodiment includes a base station (transmitter, cell, transmission point, set of transmission antennas, set of transmission antenna ports, component carrier, eNodeB) and a terminal (terminal device, mobile terminal, reception point, receiving terminal, receiver, third communication device, set of receive antennas, set of receive antenna ports, UE).

FIG. 1 is a schematic diagram illustrating a communication system for performing data transmission according to the first embodiment of the present invention. In FIG. 1, a base station 101 transmits control information and information data via a downlink 103 in order to perform data communication with a terminal 102.

The control information is subjected to an error correction coding process and so forth, and is then mapped to a control channel. The control channel subjected to a modulation process is transmitted and received via a first control channel (first physical control channel) region or a second control channel (second physical control channel) region different from the first control channel region. The term physical control channel, as used herein, is a kind of physical channel and refers to a control channel defined in a physical frame.

In terms of a point of view, the first control channel is a physical control channel that uses the same transmission port (antenna port) as a cell-specific reference signal. The second control channel is a physical control channel that uses the same transmission port as a UE-specific reference signal. The terminal 102 demodulates the control channel (first control channel) mapped to the first control channel region using the cell-specific reference signal, and demodulates the control channel (second control channel) mapped to the second control channel region using the UE-specific reference signal. The cell-specific reference signal is a reference signal common to all the terminals within a cell, and is a reference signal usable by any terminal because it is included in substantially all the resources. Accordingly, the first control channel can be demodulated by any terminal. The UE-specific reference signal is a reference signal that is included only in an allocated resource, and can be subjected to an adaptive beamforming process in a manner similar to that for data. Accordingly, adaptive beamforming gains can be obtained on the second control channel.

In terms of a different point of view, the first control channel is a physical control channel over OFDM symbols located in a front part of a physical sub-frame, and may be arranged over an entire system bandwidth (component carrier (CC)) in these OFDM symbols. The second control channel is a physical control channel over OFDM symbols located after the first control channel in the physical sub-frame, and may be arranged in part of the system bandwidth over these OFDM symbols. The first control channel is arranged over OFDM symbols dedicated to a control channel which are located in a front part of a physical sub-frame, and can thus be received and demodulated prior to rear OFDM symbols used for a physical data channel. The first control channel can also be received by a terminal that monitors only OFDM symbols dedicated to a control channel. Since the first control channel can be spread out over an entire CC, it is possible to randomize inter-cell interference. In contrast, the second control channel is arranged over rear OFDM symbols used for a shared channel (physical data channel) which terminals under communication normally receive. In addition, frequency division multiplexing allows second control channels or a second control channel and a physical data channel to be orthogonally multiplexed (multiplexed without interference).

In terms of a still different point of view, the first control channel is a cell-specific physical control channel, and is a physical channel which both a terminal in the idle state and a terminal in the connected state can acquire. The second control channel is a UE-specific physical control channel, and is a physical channel which only a terminal in the connected state can acquire. The term idle state refers to a state where data is not immediately transmitted or received, such as a state where a base station does not accumulate RRC (Radio Resource Control) information (RRC_IDLE state) or a state where a mobile station is performing discontinuous reception (DRX). The term connected state, in contrast, refers to a state where data is ready to be immediately transmitted or received, such as a state where a terminal holds network information (RRC_CONNECTED state) or a state where a mobile station is not performing discontinuous reception (DRX). The first control channel is a channel which a terminal can receive without depending on UE-specific RRC signaling. The second control channel is a channel configured using UE-specific RRC signaling, and is a channel which a terminal can receive using UE-specific RRC signaling. That is, the first control channel is a channel which any terminal can receive with pre-determined settings, and the second control channel is a channel for which UE-specific configurations can be easily changed.

FIG. 2 is a schematic block diagram illustrating a configuration of the base station 101 according to the first embodiment of the present invention. In FIG. 2, the base station 101 includes a higher layer 201, a data channel generation unit 202, a UE-specific reference signal multiplexing unit 203, a precoding unit 204, a cell-specific reference signal multiplexing unit 205, a transmit signal generation unit 206, and a transmission unit 207.

The higher layer 201 generates information data for the terminal 102, and outputs the information data to the data channel generation unit 202.

The data channel generation unit 202 performs adaptive control on the information data output from the higher layer 201 to generate a data channel for the terminal 102. Specifically, the data channel generation unit 202 performs processes such as a coding process for performing error correction coding, a scrambling process for applying a specific scrambling code to the terminal 102, a modulation process for using a multi-level modulation scheme and so forth, and a layer mapping process for performing spatial multiplexing such as MIMO. Furthermore, in a case where the data channel generation unit 202 is to map a control channel to a second control channel region described below, the control channel is multiplexed to the data channel.

The UE-specific reference signal multiplexing unit 203 generates UE-specific reference signals specific to the terminal 102 (data channel demodulation reference signal, DM-RS (Demodulation Reference Signal), DRS (Dedicated Reference Signal), Precoded RS, user-specific reference signal, UE-specific RS), and multiplexes the UE-specific reference signals to the data channel generated by the data channel generation unit 202.

The precoding unit 204 performs a precoding process specific to the terminal 102 on the data channel and UE-specific reference signals output from the UE-specific reference signal multiplexing unit 203. In the precoding process, preferably, phase rotation and so forth are performed on a signal to be generated so as to allow the terminal 102 to efficiently receive the signal (for example, maximize the receive power, reduce interference from neighboring cells, and/or reduce interference with neighboring cells). In addition, processes that can be used include, but not limited to, processes based on predetermined precoding matrices, CDD (Cyclic Delay Diversity), and transmit diversity (such as SFBC (Spatial Frequency Block Code), STBC (Spatial Time Block Code), TSTD (Time Switched Transmission Diversity), and FSTD (Frequency Switched Transmission Diversity))). In a case where a plurality of separate types of PMIs are fed back, the plurality of PMIs are subjected to computation such as multiplication and precoding can be performed.

The UE-specific reference signals are implemented using signals which are known by both the base station 101 and the terminal 102. The data channel and the UE-specific reference signals are subjected to the precoding process specific to the terminal 102 by the precoding unit 204. Accordingly, when demodulating the data channel, the terminal 102 can estimate the channel state in the downlink 103 and a channel for equalizing precoding weights used by the precoding unit 204, by using the UE-specific reference signals. That is, the base station 101 can demodulate the signals which have been subjected to the precoding process, without the need to notify the terminal 102 of the precoding weights used by the precoding unit 204.

The cell-specific reference signal multiplexing unit 205 generates cell-specific reference signals that are known by both the base station 101 and the terminal 102 (channel state measurement reference signal, CRS (Common RS), Cell-specific RS, Non-precoded RS) to measure the channel state of the downlink 103 between the base station 101 and the terminal 102. The generated cell-specific reference signals are multiplexed to the data channel and UE-specific reference signals subjected to the precoding process by the precoding unit 204.

The cell-specific reference signals may be any signal (sequence) as long as they are signals that are known by both the base station 101 and the terminal 102. The cell-specific reference signals may be implemented using, for example, a random number or a pseudo-noise sequence based on a pre-assigned parameter such as a number (cell ID) specific to the base station 101. A method for performing orthogonalization between antenna ports, such as a method for setting a resource element to which a channel state measurement reference signal is mapped to null (zero) between the antenna ports, a method for performing code division multiplexing using a pseudo-noise sequence, or a combination thereof, may be used. The channel state measurement reference signal may not necessarily be multiplexed to all the sub-frames, but may be multiplexed to only some sub-frames.

The cell-specific reference signals are reference signals to be multiplexed after the precoding process has been performed by the precoding unit 204. Thus, the terminal 102 can measure the channel state of the downlink 103 between the base station 101 and the terminal 102 using the cell-specific reference signals, and can demodulate a signal that has not yet been subjected to the precoding process by the precoding unit 204.

The transmit signal generation unit 206 maps the signals output from the cell-specific reference signal multiplexing unit 205 to the respective resource elements of the antenna ports. Specifically, the transmit signal generation unit 206 maps the data channel to a shared channel (PDSCH; Physical Downlink Shared Channel) region described below, and maps the control channel to be transmitted through the second control channel region to the second control channel region. Further, when mapping a control channel to a first control channel (PDCCH; Physical Downlink Control Channel) region described below, the transmit signal generation unit 206 multiplexes the control channel to the signals output from the cell-specific reference signal multiplexing unit 205. Here, the base station 101 can map control channels addressed to a plurality of terminals to the first control channel region or the second control channel region.

The transmission unit 207 performs processes such as inverse fast Fourier transform (IFFT), the addition of guard interval, and conversion into radio frequencies, and then transmits the resulting signals via transmission antennas, where the number of transmission antennas (the number of transmission antenna ports) is at least one.

FIG. 3 is a schematic block diagram illustrating a configuration of the terminal 102 according to the first embodiment of the present invention. In FIG. 3, the terminal 102 includes a reception unit 301, a reception signal processing unit 302, a control channel processing unit 303, a data channel processing unit 304, and a higher layer 305.

The reception unit 301 receives signals transmitted from the base station 101 using receive antennas, where the number of receive antennas (the number of receive antenna ports) is at least one, and performs a process for conversion from radio frequencies to baseband signals, the removal of the added guard interval, and a time-frequency conversion process based on fast Fourier transform (FFT) or the like.

The reception signal processing unit 302 de-maps (de-multiplexes) the signals mapped by the base station 101. Specifically, the reception signal processing unit 302 de-maps the first control channel and/or second control channel mapped to the first control channel and/or the second control channel region, and the data channel mapped to the data channel region.

The control channel processing unit 303 searches for and detects a control channel mapped to the first control channel region or the second control channel region and addressed to the terminal 102. The control channel processing unit 303 configures the first control channel region or the second control channel region as a control channel region in which the control channel is searched for. The method for setting the control channel region is determined by whether the base station 101 configures the second control channel for the terminal 102 through UE-specific configuration information on the second control channel, which is higher-layer control information (for example, RRC (Radio Resource Control) signaling) of which the terminal 102 is notified.

That is, in a case where the base station 101 notifies the terminal 102 of the UE-specific configuration information of the second control channel and configures the second control channel, the terminal 102 searches for and detects the control channel mapped to the second control channel and addressed to the terminal 102. On the other hand, in a case where the base station 101 does not notify the terminal 102 of the UE-specific configuration information on the second control channel or does not configure the second control channel, the terminal 102 searches for and detects the control channel mapped to the first control channel and addressed to the terminal 102.

The control channel processing unit 303 uses the UE-specific reference signals for the demodulation of the control channel mapped to the second control channel region and addressed to the terminal 102. The control channel processing unit 303 uses the cell-specific reference signals for the demodulation of the control channel mapped to the first control channel region and addressed to the terminal 102.

Further, the control channel processing unit 303 searches for and identifies the control channel addressed to the terminal 102 in the configured control channel region. Specifically, the control channel processing unit 303 sequentially searches all or some of the control channel candidates obtained in accordance with the type of control information, the position of the resource to be mapped, the size of the resource to be mapped, and so forth, by performing a demodulation and decoding process. The control channel processing unit 303 determines whether control information is the control information addressed to the terminal 102, by using error detection codes (for example, CRC (Cyclic Redundancy Check) codes) attached to the control information. This search method is also called blind decoding.

The reception signal processing unit 302 identifies the detected control channel. If the de-mapped data channel includes the data channel addressed to the terminal 102, the reception signal processing unit 302 outputs the data channel to the data channel processing unit 304. A control information signal is shared with the terminal 102 (also including the higher layer), and used for various kinds of control to be performed by the terminal 102, such as the demodulation of the data channel.

The data channel processing unit 304 performs processes, such as a channel estimation process, a channel compensation process (filtering process), a layer de-mapping process, a demodulation process, a descrambling process, and a decoding process, on the input data channel, and outputs the result to the higher layer 305. In the channel estimation process, amplitude and phase variations (frequency response, transfer function) in each resource element for each layer (rank, spatial multiplexing) are estimated (channel estimation) in accordance with the UE-specific reference signals multiplexed to the input data channel to determine channel estimates. For a resource element to which no UE-specific reference signals are mapped, channel estimation is performed using interpolation in the frequency direction and the time direction based on a resource element to which a UE-specific reference signal is mapped. In the channel compensation process, channel compensation is performed on the input data channel using the estimated channel estimates to detect (extract) the data channel for each layer. The detection method may be implemented using ZF (Zero Forcing)-based and/or MMSE (Minimum Mean Square Error)-based equalization, removal of interference, or the like. In the layer de-mapping process, signals for individual layers are de-mapped to obtain the respective codewords. Subsequently, the process is performed on a codeword-by-codeword basis. In the demodulation process, demodulation is based on the modulation scheme used. In the descrambling process, descrambling is based on the scrambling codes used. In the decoding process, an error correction decoding process is based on the coding method applied.

FIG. 4 is a diagram illustrating an example of one resource block pair that the base station 101 maps. FIG. 4 illustrates two resource blocks (a resource block pair), and each resource block is composed of twelve subcarriers in the frequency direction, and seven OFDM symbols in the time direction. Each subcarrier for a duration of one OFDM symbol is called a resource element. The resource block pairs are arranged in the frequency direction, and the number of resource block pairs can be configured for each base station. For example, the number of resource block pairs can be set to 6 to 110. The width of the resource block pairs in the frequency direction is called a system bandwidth. A resource block pair in the time direction is called a sub-frame. In each sub-frame, consecutive sets of seven OFDM symbols in the time direction are each also called a slot. In the following description, resource block pairs are also referred to simply as resource blocks.

Among the resource elements shown shaded, R0 to R1 represent cell-specific reference signals for antenna ports 0 to 1, respectively. The cell-specific reference signals illustrated in FIG. 4 are used in the case of two antenna ports, the number of which can be changed. For example, cell-specific reference signals for one antenna port or four antenna ports can be mapped. Cell-specific reference signals can be set for up to four antenna ports (antenna ports 0 to 3).

Among the resource elements shown shaded, D1 to D2 represent UE-specific reference signals in CDM (Code Division Multiplexing) group 1 to CDM group 2, respectively. The UE-specific reference signals in CDM group 1 and CDM group 2 are each subjected to CDM using orthogonal codes such as Walsh codes. The UE-specific reference signals in CDM group 1 and CDM group 2 are further mutually subjected to FDM (Frequency Division Multiplexing). The UE-specific reference signals can be mapped to up to rank 8 using eight antenna ports (antenna ports 7 to 14) in accordance with the control channel or data channel to be mapped to the resource block pair. In addition, the UE-specific reference signals are configured such that the spreading code length for CDM and/or the number of resource elements to be mapped can be changed in accordance with the rank for mapping.

For example, the UE-specific reference signals for ranks 1 to 2 are formed of spreading codes of 2-chip length for antenna ports 7 to 8, and are mapped to CDM group 1. The UE-specific reference signals for ranks 3 to 4 are formed of spreading codes of 2-chip length for antenna ports 9 to 10 in addition to antenna ports 7 to 8, and are further mapped to CDM group 2. The UE-specific reference signals for ranks 5 to 8 are formed of spreading codes of 4-chip length for antenna ports 7 to 14, and are mapped to CDM group 1 and CDM group 2.

In the UE-specific reference signals, a scrambling code is further multiplied on an orthogonal code on each antenna port. The scrambling code is generated based on the cell ID and the scrambling ID which are sent from the base station 101. For example, a scrambling code is generated from a pseudo-noise sequence generated based on the cell ID and the scrambling ID which are notified by the base station 101. The scrambling ID is, for example, a value representing 0 or 1. The scrambling IDs and antenna ports to be used can also be subjected to joint coding, and information indicating them can also be formed into an index.

Among the resource elements shown shaded, the area composed of the top first to third OFDM symbols is set as an area where the first control channel is to be mapped. In addition, the number of OFDM symbols in the area where the first control channel is to be mapped can be configured for each sub-frame. The resource elements in a solid white color represent an area where the second control channel or the shared channel is to be arranged. The area where the second control channel or the shared channel is to be arranged can be set for each resource block pair. The rank of the control channel to be mapped to the second control channel and/or the data channel to be mapped to the shared channel can be set different from the rank of the control signal to be mapped to the first control channel.

The number of resource blocks can be changed in accordance with the frequency bandwidth (system bandwidth) used in the communication system. For example, 6 to 110 resource blocks can be used, the unit of which is also called a component carrier. A base station can further configure a plurality of component carriers for a terminal by using frequency aggregation. For example, a base station may configure five component carriers contiguous and/or non-contiguous in the frequency direction for a terminal, where the bandwidth of each component carrier is 20 MHz, thereby totaling a bandwidth of 100 MHz which can be supported by the communication system.

FIG. 5 is a diagram illustrating an example of channels that the base station 101 maps. In the case illustrated in FIG. 5, a frequency bandwidth of 12 resource block pairs is used as the system bandwidth. The first control channel, or PDCCH, is mapped in the top first to third OFDM symbols in a sub-frame. The first control channel is distributed over the system bandwidth in the frequency direction. The shared channel is arranged in the OFDM symbols other than the OFDM symbols for the first control channel in the sub-frame.

The details of the configuration of the PDCCH will now be described. The PDCCH is composed of a plurality of control channel elements (CCEs). The number of CCEs used in each downlink component carrier depends on the downlink component carrier bandwidth, the number of OFDM symbols constituting the PDCCH, and the number of transmission ports for downlink reference signals which depends on the number of transmission antennas in the base station used for communication. Each CCE is composed of a plurality of downlink resource elements (resources each defined by one OFDM symbol and one subcarrier).

CCEs used between a base station and a terminal are assigned respective numbers to identify the CCEs. The numbering of the CCEs is based on a predetermined rule. Here, CCE_t denotes the CCE having the CCE number t. The PDCCH is constituted by an aggregation of a plurality of CCEs (CCE Aggregation). The number of CCEs in this aggregation is referred to as “CCE aggregation level”. The CCE aggregation level in the PDCCH is set in the base station in accordance with a coding rate set for the PDCCH and/or the number of bits in a DCI included in the PDCCH. The combination of CCE aggregation levels which can be possibly used for the terminal is determined in advance. An aggregation of n CCEs is referred to as “CCE aggregation level n”.

One resource element group is composed of four neighboring downlink resource elements in the frequency domain. Each CCE is composed of nine different resource element groups that are scattered in the frequency domain and the time domain. Specifically, all the resource element groups assigned numbers for the entire downlink component carrier are interleaved in units of resource element groups using a block interleaver, and nine resource element groups having contiguous numbers, which have been interleaved, constitute one CCE.

An area (search space, retrieval area) in which a PDCCH is searched for, called an SS (Search Space), is set for each terminal. Each SS is composed of a plurality of CCEs. The CCEs are assigned numbers in advance, and the SS is composed of a plurality of CCEs having contiguous numbers. The number of CCEs constituting a certain SS is determined in advance. An SS for each CCE aggregation level is composed of an aggregate of a plurality of PDCCH candidates. SSs are classified into a cell-specific search space CSS (Cell-specific SS) for which, among the included CCEs, the CCE number of the CCE having the smallest number is shared in a cell, and a UE-specific search space USS (UE-specific SS) for which the CCE number of the CCE having the smallest number is UE-specific. In the CSS, a PDCCH to which control information to be read by a plurality of terminals, such as system information and information concerning paging, is assigned (included), or a PDCCH to which a downlink/uplink grant indicating instructions for a fallback to a low-level transmit scheme or random access is assigned (included) can be arranged.

A base station transmits a PDCCH using one or more CCEs in an SS set in a terminal. The terminal decodes a received signal using one or more CCEs in the SS, and performs a process for detecting the PDCCH addressed to the terminal (referred to as blind decoding). The terminal sets different SSs for the respective CCE aggregation levels. Thereafter, the terminal performs blind decoding using predetermined combinations of CCEs in the different SSs for the respective CCE aggregation levels. In other words, the terminal attempts blind decoding on each of the PDCCH candidates in the different SSs for the respective CCE aggregation levels. The above-described series of processes performed by the terminal is referred to as PDCCH monitoring.

The second control channel (X-PDCCH, PDCCH on PDSCH, Extended PDCCH) is mapped in the OFDM symbols other than the OFDM symbols for the first control channel. The second control channel and the shared channel are arranged in different resource blocks. Resource blocks in which the second control channel and the shared channel may be arranged are configured for each terminal. The starting position for the OFDM symbols in which the second control channel is to be arranged can be determined using a method similar to that for the shared channel. That is, this method can be realized by the base station 101 configuring some resources in the first control channel as a PCFICH (Physical control format indicator channel), and mapping information indicating the number of OFDM symbols for the first control channel.

The starting position for the OFDM symbols in which the second control channel is to be arranged is defined in advance, and can be set to, for example, the top fourth OFDM symbol in the sub-frame. In this case, if the number of OFDM symbols for the first control channel is less than or equal to 2, the second to third OFDM symbols in the resource block pair where the second control channel is to be arranged are set to null without mapping a signal. Other control channels or data channels can further be mapped to the resources set to null. The starting position for the OFDM symbols constituting the second control channel can be configured through higher-layer control information. The sub-frame illustrated in FIG. 5 is time-multiplexed, and the second control channel can be configured for each sub-frame.

As an SS in which the X-PDCCH is searched for, an SS can be composed of a plurality of CCEs similarly to the PDCCH. That is, a resource element group is composed of a plurality of resource elements in an area that is set as an area for the second control channel illustrated in FIG. 5, and furthermore a CCE is composed of a plurality of resource elements. Accordingly, similarly to the case of the PDCCH described above, an SS in which the X-PDCCH is searched for (monitored) can be formed.

Alternatively, as an SS in which the X-PDCCH is searched for, unlike the PDCCH, an SS can be composed of one or more resource blocks. That is, an SS may be composed of an aggregation of one or more resource blocks (RB Aggregation), in units of resource blocks in an area that is set as the area for the second control channel illustrated in FIG. 5. The number of RBs included in this aggregation is referred to as an “RB aggregation level”. An SS is composed of a plurality of RBs having contiguous numbers, starting from the RB having the smallest number, and the number of one or more RBs having contiguous numbers is determined in advance. An SS for each RB aggregation level is composed of an aggregate of a plurality of X-PDCCH candidates.

A base station transmits an X-PDCCH using one or more RBs in an SS set in a terminal. The terminal decodes a received signal using one or more RBs in the SS, and performs a process for detecting the X-PDCCH addressed to the terminal (performs blind decoding). The terminal sets different SSs for the respective RB aggregation levels. Thereafter, the terminal performs blind decoding using predetermined combinations of RBs in the different SSs for the respective RB aggregation levels. In other words, the terminal performs blind decoding on each of the X-PDCCH candidates in the different SSs for the respective RB aggregation levels (monitors the X-PDCCH).

In a case where the base station 101 is to notify the terminal 102 of a control channel via the second control channel region, the base station 101 configures the monitoring of the second control channel for the terminal 102, and maps the control channel for the terminal 102 to the second control channel region. In a case where the base station 101 is to notify the terminal 102 of a control channel via the first control channel region, the base station 101 maps the control channel for the terminal 102 to the first control channel region without configuring the monitoring of the second control channel for the terminal 102.

On the other hand, in a case where the monitoring of the second control channel is configured by the base station 101, the terminal 102 performs blind decoding of the control channel addressed to the terminal 102 in the second control channel region. In a case where the monitoring of the second control channel is not configured by the base station 101, the terminal 102 does not perform blind decoding of the control channel addressed to the terminal 102 in the second control channel.

Hereinafter, a description will be given of the control channel to be mapped to the second control channel region. The control channel to be mapped to the second control channel region is processed for each piece of control information on one terminal, and is subjected to processes such as, similarly to the data channel, a scrambling process, a modulation process, a layer mapping process, and a precoding process. The control channel to be mapped to the second control channel region is subjected to a precoding process specific to the terminal 102 together with the UE-specific reference signal. In this case, the precoding process is preferably performed with precoding weights suitable for the terminal 102.

In a case where an SS is composed of one or more resource blocks, the control channel to be mapped to the second control channel region can be mapped so as to include different kinds of control information for the front slot (first slot) and the rear slot (second slot) in a sub-frame. For example, a control channel including allocation information (downlink allocation information) for a downlink shared channel to be transmitted from the base station 101 to the terminal 102 is mapped to the front slot in a sub-frame. On the other hand, a control channel including allocation information (uplink allocation information) for an uplink shared channel to be transmitted from the terminal 102 to the base station 101 is mapped to the rear slot in the sub-frame. Alternatively, a control channel including uplink allocation information for the terminal 102 with respect to the base station 101 may be mapped to the front slot in a sub-frame, and a control channel including downlink allocation information for the base station 101 with respect to the terminal 102 may be mapped to the rear slot in the sub-frame.

A data channel for the terminal 102 or any other terminal may be mapped to the front and/or rear slot for the second control channel. Alternatively, a control channel for the terminal 102 or a terminal for which the second control channel has been set (including the terminal 102) may be mapped to the front and/or rear slot for the second control channel.

The base station 101 multiplexes the UE-specific reference signal to the control channel to be mapped to the second control channel region. The terminal 102 performs a demodulation process on the control channel to be mapped to the second control channel region, by using the UE-specific reference signals to be multiplexed. UE-specific reference signals for some or all the antenna ports 7 to 14 are used. In this case, the control channel to be mapped to the second control channel region can be MIMO-transmitted using a plurality of antenna ports.

For example, a UE-specific reference signal in the second control channel is transmitted using a predefined antenna port and a scrambling code. Specifically, a UE-specific reference signal in the second control channel is generated using antenna port 7, which is defined in advance, and a scrambling ID.

For example, a UE-specific reference signal in the second control channel is generated using an antenna port and a scrambling ID which are sent via RRC signaling or PDCCH signaling. Specifically, either antenna port 7 or antenna port 8 is notified as the antenna port to be used for a UE-specific reference signal in the second control channel via RRC signaling or PDCCH signaling. Any value of 0 to 3 is notified as the scrambling ID to be used for the UE-specific reference signal in the second control channel via RRC signaling or PDCCH signaling.

In the following, as an example of a method in which the base station 101 configures the second control channel for the terminal 102 (a method for configuration of the second control channel region and a method for configuration of the monitoring of the second control channel), the configuration of the second control channel region and the configuration of a transmission mode implicitly indicate the configuration of the monitoring of the second control channel. The base station 101 notifies the terminal 102 of UE-specific configuration information for radio resources (RadioResourceConfigDedicated) through higher-layer control information, thereby configuring the second control channel. The UE-specific configuration information for radio resources is control information used for the configuration/reconfiguration/release of resource blocks, the UE-specific configurations for the physical channel, and so forth.

FIG. 6 is a diagram illustrating the flow for setting the UE-specific configuration information for radio resources. The base station 101 notifies the terminal 102 of the UE-specific configuration information for radio resources. The terminal 102 performs UE-specific configuration for radio resources in accordance with the UE-specific configuration information for radio resources sent from the base station 101, and notifies the base station 101 of the completion of the configuration of the UE-specific configuration information for radio resources.

FIG. 7 is a diagram illustrating an example of the UE-specific configuration information for radio resources. The UE-specific configuration information for radio resources includes UE-specific configuration information for the physical channel (PhysicalConfigDedicated). The UE-specific configuration information for the physical channel is control information for defining UE-specific configurations for the physical channel. The UE-specific configuration information for the physical channel includes configuration information on channel state reports (CQI-ReportConfig), UE-specific configuration information on antenna information (AntennaInfoDedicated), and UE-specific configuration information on the second control channel (XPDCCH-ConfigDedicated). The configuration information on channel state reports is used to define configuration information for reporting the channel state in the downlink 103. The UE-specific configuration information on antenna information is used to define UE-specific antenna information in the base station 101. The UE-specific configuration information on the second control channel is used to define UE-specific configuration information on the second control channel.

The configuration information on channel state reports includes configuration information on aperiodic channel state reports (cqi-ReportModeAperiodic), and configuration information on periodic channel state reports (CQI-ReportPeriodic). The configuration information on aperiodic channel state reports is configuration information for aperiodically reporting the channel state in the downlink 103 via an uplink shared channel (PUSCH; Physical Uplink Shared Channel). The configuration information of the periodic channel state report is configuration information for periodically reporting the channel state in the downlink 103 via an uplink control channel (PUCCH; Physical Uplink Control Channel).

The UE-specific configuration information on antenna information includes a transmission mode. The transmission mode is information indicating a transmission method by which the base station 101 communicates with the terminal 102. For example, the transmission mode is defined as any of transmission modes 1 to 10 in advance. Transmission mode 1 is a transmission mode based on a single-antenna port transmission scheme in which antenna port 0 is used. Transmission mode 2 is a transmission mode based on a transmit diversity scheme. Transmission mode 3 is a transmission mode based on a cyclic delay diversity scheme. Transmission mode 4 is a transmission mode based on a closed-loop spatial multiplexing scheme. Transmission mode 5 is a transmission mode based on a multi-user MIMO scheme. Transmission mode 6 is a transmission mode based on a closed-loop spatial multiplexing scheme in which a single antenna port is used. Transmission mode 7 is a transmission mode based on a single-antenna port transmission scheme in which antenna port 5 is used. Transmission mode 8 is a transmission mode based on a closed-loop spatial multiplexing scheme in which antenna ports 7 to 8 are used. Transmission mode 9 is a transmission mode based on a closed-loop spatial multiplexing scheme in which antenna ports 7 to 14 are used. Transmission modes 1 to 9 are also referred to as first transmission modes.

Transmission mode 10 is defined as a transmission mode different from transmission modes 1 to 9. For example, transmission mode 10 can be a transmission mode based on a CoMP scheme. Here, extensions with the employment of the CoMP scheme include the optimization of channel state reports, improvements in accuracy (for example, the employment of information suitable for CoMP communication, such as precoding information and phase difference information between base stations), and so forth. Transmission mode 10 can also be a transmission mode based on a communication scheme which is an extended (enhanced) version of the multi-user MIMO scheme and which can be achieved using the communication schemes specified in transmission modes 1 to 9. The extended version of the multi-user MIMO scheme includes the optimization of channel state reports and improvements in accuracy (for example, the employment of information suitable for multi-user MIMO communication, such as CQI (Channel Quality Indicator) information), improvements in orthogonality between terminals to be multiplexed to the same resource, and so forth.

Transmission mode 10 can also be a transmission mode based on a CoMP scheme and/or an extended multi-user MIMO scheme in addition to all or some of the communication schemes specified in transmission modes 1 to 9. For example, transmission mode 10 can be a transmission mode based on a CoMP scheme and/or an extended multi-user MIMO scheme in addition to the communication scheme specified in transmission mode 9. Alternatively, transmission mode 10 can be a transmission mode in which a plurality of channel state measurement reference signals (CSI-RS; Channel State Information-RS) can be configured. Transmission mode 10 is also referred to as a second transmission mode.

A base station can communicate with a terminal configured to transmission mode 10 in which a plurality of transmission schemes can be used, without notifying the terminal of which of the plurality of transmission schemes was used when transmitting the data channel. That is, when receiving the data channel, the terminal can perform communication without being notified of which of the plurality of transmission schemes was used even if the terminal is configured to transmission mode 10 in which a plurality of transmission schemes can be used.

The second transmission mode is a transmission mode in which the second control channel can be configured. That is, in a case where the base station 101 configures one of the first transmission modes for the terminal 102, the base station 101 maps the control channel for the terminal 102 to the first control channel region. In a case where the base station 101 configures the second transmission mode for the terminal 102, the base station 101 maps the control channel for the terminal 102 to the first control channel region or the second control channel region. On the other hand, in a case where the terminal 102 is configured to one of the first transmission modes by the base station 101, the terminal 102 performs blind decoding on the first control channel. In a case where the terminal 102 is configured to the second transmission mode by the base station 101, the terminal 102 performs blind decoding on one of the first control channel and the second control channel.

In a case where the terminal 102 is configured to the second transmission mode, the terminal 102 switches (selects) the control channel to be subjected to blind decoding in accordance with whether the UE-specific configuration information on the second control channel has been configured by the base station 101. That is, in a case where the terminal 102 is configured to the second transmission mode and the UE-specific configuration information on the second control channel is configured by the base station 101, the terminal 102 performs blind decoding of the first control channel and the second control channel. In a case where the terminal 102 is configured to the second transmission mode and the UE-specific configuration information on the second control channel is not configured by the base station 101, the terminal 102 performs blind decoding of only the first control channel.

Furthermore, in a case where the terminal 102 is configured to the second transmission mode and is then configured to one of the first transmission modes by the base station 101 after the UE-specific configuration information on the second control channel has been configured, the terminal 102 performs blind decoding of the first control channel. In a case where the terminal 102 is configured to the second transmission mode and is then configured to some or all the first transmission modes by the base station 101 after the UE-specific configuration information on the second control channel has been configured, the terminal 102 may perform blind decoding of the first control channel and the second control channel.

Transmission modes (second transmission mode) in which the second control channel can be configured may include some or all the transmission modes in which the UE-specific reference signals can be used, and may include, for example, transmission modes 8 to 10. Transmission modes (first transmission mode) in which only the first control channel can be configured may include some or all the transmission modes in which it is not possible to use the UE-specific reference signals, and may include, for example, transmission modes 1 to 7.

FIG. 8 is a diagram illustrating an example of the UE-specific configuration information on the second control channel. The UE-specific configuration information on the second control channel includes sub-frame configuration information on the second control channel (XPDCCH-SubframeConfig-r11). The sub-frame configuration information on the second control channel is used to define sub-frame information for configuration of the second control channel. The sub-frame configuration information on the second control channel includes a sub-frame configuration pattern (subframeConfigPattern-r11), and second control channel configuration information (xpdcch-Config-r11).

The sub-frame configuration pattern is information indicating a sub-frame in which the second control channel is to be configured. For example, the sub-frame configuration pattern is n-bit bitmap information. The information contained in each bit indicates whether the current sub-frame is a sub-frame to be configured as a second control channel. That is, in the sub-frame configuration pattern, n sub-frames can be set as a period. In this case, certain sub-frames to which a synchronization signal, a broadcast channel, and so forth are to be mapped can be excluded. Specifically, the remainder of dividing the sub-frame number defined in each sub-frame by n corresponds to each bit in the sub-frame configuration pattern. For example, n is set in advance to a value such as 8 or 40. If the information for a certain sub-frame in the sub-frame configuration pattern is “1”, a second control channel of this sub-frame is configured. If the information for a certain sub-frame in the sub-frame configuration pattern is “0”, a second control channel of this sub-frame is not configured.

Certain sub-frames to which a synchronization signal for synchronizing the terminal 102 with the base station 101, a broadcast channel for broadcasting control information on the base station 101, and so forth are to be mapped may be prevented from being configured as a second control channel in advance.

In another example of the sub-frame configuration pattern, a pattern of sub-frames to be configured as a second control channel is formed as an index in advance, and information indicating the index is defined as a sub-frame configuration pattern.

The second control channel configuration information includes resource allocation type (resourceAllocationType-r11), resource allocation information (resourceBlockAssignment-r11), OFDM symbol starting position (xpdcch-Start-r11), and response signal control information (pucch-Config-r11).

The resource allocation type is information indicating a format (type) of information specifying a resource block to be configured as a second control channel in a sub-frame. The resource allocation information is information specifying a resource block to be configured as a second control channel, and is defined based on the format of the resource allocation type.

For example, the resource allocation type can define types 0 to 2. If the resource allocation type is type 0, the resource allocation information is bitmap information that can be assigned to each resource block group defined in units of a plurality of contiguous resource blocks. The number of resource blocks in a resource block group can be defined in accordance with the system bandwidth. If the resource allocation type is type 1, the resource allocation information is bitmap information that can be assigned to each resource block in a plurality of resource block group subsets, where each resource block group subset is defined in a plurality of subsets in units of resource block groups. The resource allocation information also includes information indicating a selected resource block group subset. If the resource allocation type is type 1, the resource allocation information is information indicating a resource block with which allocation starts in contiguous resource blocks, and information indicating the number of resource blocks to be allocated.

The OFDM symbol starting position is information indicating the position of the OFDM symbol with which the second control channel starts in a sub-frame. For example, the OFDM symbol starting position indicates any of 1 to 3. As in the description with reference to FIG. 5, the OFDM symbol starting position for the second control channel may be identified through PCFICH. The OFDM symbol starting position for the second control channel can be defined in advance, in which case the OFDM symbol starting position may not necessarily be notified.

The response signal control information is resource allocation information for the uplink control channel for giving notification of a response signal (for example, ACK (Acknowledge), NACK (Negative ACK), or the like) indicating whether the terminal 102 has correctly received the data channel specified in the control information sent via the second control channel.

As described above, in a case where the base station 101 is to set the second control channel, the base station 101 notifies the terminal 102 of the UE-specific configuration information for radio resources, which includes the UE-specific configuration information on the second control channel, via RRC signaling. Also, in a case where the base station 101 is to change the configured second control channel, the base station 101 notifies the terminal 102 of the UE-specific configuration information for radio resources, which includes the UE-specific configuration information on the second control channel whose parameters have been changed, via RRC signaling. Also, in a case where the base station 101 is to release the configured second control channel, the base station 101 notifies the terminal 102 via RRC signaling. For example, the base station 101 sends the UE-specific configuration information for radio resources, which does not include the UE-specific configuration information on the second control channel. Alternatively, the base station 101 may send control information for releasing the UE-specific configuration information on the second control channel.

FIG. 9 is a diagram illustrating the flow for a process for receiving the control channel and the data channel at the terminal 102. In step S102, the terminal 102 receives UE-specific configuration information for radio resources via RRC signaling. In step S103, the terminal 102 identifies the received UE-specific configuration information for radio resources, and performs a UE-specific setting process for the radio resources. In step S104, the terminal 102 determines whether the UE-specific configuration information for radio resources includes UE-specific configuration information on the second control channel.

If UE-specific configuration information on the second control channel is included (YES in step S104), in step S106, the terminal 102 identifies the UE-specific configuration information on the second control channel, and configures the second control channel. In step S107, the terminal 102 searches for (blind decoding) and detects the control channel addressed to the terminal 102 in the first control channel region and the configured second control channel region.

On the other hand, if UE-specific configuration information on the second control channel is not included (NO in step S104), in step S105, the terminal 102 searches for (blind decoding) and detects the control channel addressed to the terminal 102 in the first control channel set in advance. In step S108, the terminal 102 identifies the detected control channel. In step S108, if the terminal 102 has failed to detect the control channel addressed to the terminal 102, the receiving process for the current sub-frame ends. In step S109, the terminal 102 performs setting for receiving the data channel in accordance with the detected control channel, and receives the data channel.

With the use of the method described above, the base station 101 can efficiently notify the terminal 102 of control information. That is, the base station 101 can map the control channel for the terminal 102 to the first control channel region or the second control channel region. Accordingly, the base station 101 can perform efficient resource allocation scheduling of the control channel of which a plurality of terminals are notified. In addition, the terminal 102 can be mapped with the control channel by the base station 101 via the first control channel region or the second control channel region. Accordingly, the terminal 102 can reduce the number of candidates in which the control channel is searched for, and can perform an efficient receiving process.

In the above example, a control channel for a terminal set to any of transmission modes 1 to 10 can be mapped to the first control channel region, and a control channel for a terminal set to transmission mode 10 can be mapped to the second control channel region. That is, the base station 101 can notify the terminal 102 of a control channel via the first control channel region regardless of the transmission mode to be configured for the terminal 102. Furthermore, in a case where the base station 101 is to configure transmission mode 10 for the terminal 102, the base station 101 can notify the terminal 102 of a control channel via the second control channel region. Accordingly, the base station 101 can perform resource allocation scheduling taking a communication scheme achievable with transmission mode 10 into account.

Particularly in transmission mode 10 in which the second control channel can be configured, the base station 101 can implement a CoMP communication scheme, a multi-user MIMO communication scheme, and so forth for the terminal 102, and can thus perform resource allocation scheduling taking these communication schemes into account. Since the first control channel can be configured for all terminals, the base station 101 can maintain backward compatibility with terminals for which transmission mode 10 is difficult to configure. The notification of the control channel via the first control channel region can be achieved without configuring the second control channel, and therefore the overhead of the control information in RRC signaling can be reduced.

The base station 101 notifies the terminal 102 of the control channel while switching between only the first control channel to be used and the second control channel to be used, and the terminal 102 switches the control channel region in which the control channel is monitored (blind decoding is performed) in accordance with the instruction of the base station 101. More specifically, the base station 101 determines whether the second control channel region is to be used for the transmission of the control channel addressed to the terminal 102, and explicitly or implicitly notifies the terminal 102. Further, the base station 101 sends the settings of the second control channel to the terminal 102 through signaling. Upon being explicitly or implicitly notified of the use of the second control channel by the base station 101, the terminal 102 monitors the second control channel and the first control channel in accordance with the settings of the second control channel. If the terminal 102 is not explicitly or implicitly notified of the use of the second control channel by the base station 101, the terminal 102 monitors only the first control channel.

The switching of the control channel is controlled in accordance with an explicit or implicit notification from the base station 101 to the terminal 102 as to whether the second control channel is to be used. In the foregoing description, the description has been given of, but not limited to, an example in which notification as to whether the second control channel is to be used is implicitly given in accordance with the transmission mode sent from the base station 101 to the terminal 102 and control information for setting the second control channel.

For example, an agreement that only the first control channel is used in the first transmission mode and both the first control channel and the second control channel can be used in the second transmission mode is established in advance between the base station 101 and the terminal 102. Further, the base station 101 notifies the terminal 102 of the second control channel configuration information. Then, the base station 101 notifies the terminal 102 of a transmission mode, thereby implicitly notifying whether the second control channel is to be used. The base station 101 notifies the terminal 102 of second control channel region configuration information, and the terminal 102, which has been notified of the second transmission mode, monitors the first control channel and the second control channel. The terminal 102, which has been notified of the first transmission mode, monitors only the first control channel. That is, the control channel to be monitored by the terminal 102 is implicitly switched in accordance with the transmission mode of which the base station 101 notifies the terminal 102.

In another example, an agreement that only the first control channel is used in a case where the second control channel configuration information (such as configuration information on the second control channel region) is not included even once in signaling such as RRC signaling or in a case where the second control channel configuration information is not configured in the terminal 102 by releasing the second control channel configuration information, and the second control channel can be used in a case where the second control channel configuration information is notified through signaling and in a case where the second control channel configuration information is configured in the terminal 102 is established in advance between the base station 101 and the terminal 102. Then, whether the second control channel is to be used is implicitly notified depending on whether the second control channel configuration information is to be configured through signaling from the base station 101 to the terminal 102. In a case where the base station 101 sets the second control channel configuration information in the terminal 102, the terminal 102 monitors the first control channel and the second control channel. In a case where the base station does not set the second control channel configuration information in the terminal 102, the terminal 102 monitors only the first control channel without monitoring the second control channel. That is, the control channel to be monitored by the terminal 102 is implicitly switched in accordance with configuration information for setting the second control channel, which is control information of which the base station 101 notifies the terminal 102.

In still another example, the base station 101 notifies the terminal 102 of the second control channel region configuration information. In addition, the base station 101 explicitly notifies the terminal 102 of configuration information indicating whether the second control channel is to be used, through signaling such as RRC signaling. If the configuration information indicating whether the second control channel is to be used indicates that the second control channel is to be used, the terminal 102 monitors both the first control channel and the second control channel. If the configuration information indicating whether the second control channel is to be used from the base station 101 to the terminal 102 indicates that the second control channel is not to be used, the terminal 102 monitors only the first control channel. The control channel to be monitored by the terminal 102 may be switched in accordance with whether the configuration information indicating that the second control channel is to be used has been configured.

In still another example, the base station 101 notifies the terminal 102 of the second control channel region configuration information. The base station 101 notifies the terminal 102 of the enabling/disabling of the second control channel through signaling such as physical control information. In a case where the terminal 102 is notified of control information indicating the enabling of the second control channel via the first control channel, the terminal 102 starts, in addition to the monitoring of the first control channel, the monitoring of the second control channel from the sub-frame for which the terminal 102 has been notified of the enabling of the second control channel. In a case where the terminal 102 is notified of disabling of the second control channel via the first control channel or the second control channel, the terminal 102 stops the monitoring of the second control channel from the sub-frame subsequent to the sub-frame for which the terminal 102 has been notified of the disabling of the second control channel. The enabling/disabling of the second control channel may be indicated by using the code point in certain downlink control information (such as indicating that the second control channel is enabled in a case where the bit sequence in the control information format is a certain bit sequence), or by using masking with a certain code (such as indicating that the second control channel is enabled if it is masked with a certain code).

Accordingly, the control channel can be switched in accordance with any explicit or implicit notification given by the base station 101.

In this manner, according to one point of view, the monitoring of the second control channel (the monitoring of both the first control channel and the second control channel) is configured through signaling from the base station 101 to the terminal 102. In this case, the terminal 102 monitors both the first control channel and the second control channel in one sub-frame. That is, the terminal 102 searches for the first control channel in an SS in which the first control channel is searched for within one sub-frame, and searches for the second control channel in an SS in which the second control channel is searched for.

In this case, any of the following methods may be used as an example of the SS setting method or the monitoring method.

(1) A CSS and a USS (first USS) are set in the first control channel region, and a USS (second USS) is set in the second control channel region. If the monitoring of the second control channel is not configured, a terminal searches for the first control channel in the CSS and the first USS. If the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for the first control channel in the CSS, and the terminal searches for the second control channel in the second USS. In other words, if the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for a control channel arranged by the base station in the CSS in the first control channel region and the USS in the second control channel region. Therefore, the number of physical control channels usable within one sub-frame increases. In addition, the increase in the number of blind decoding attempts can be suppressed.

(2) A CSS and a USS (first USS) are set in the first control channel region, and a USS (second USS) is set in the second control channel region. If the monitoring of the second control channel is not configured, a terminal searches for the first control channel in the CSS and the first USS. If the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for the first control channel in the CSS and the first USS, and the terminal searches for the second control channel in the second USS. In other words, if the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for a control channel arranged by the base station in the CSS in the first control channel region and the USSs in the first control channel region and the second control channel region. Therefore, the number of physical control channels usable within one sub-frame increases. In addition, the CSS can be shared by a plurality of terminals within a cell.

(3) A CSS (first CSS) and a USS (first USS) are set in the first control channel region, and a CSS (second CSS) and a USS (second USS) are set in the second control channel region. If the monitoring of the second control channel is not configured, a terminal searches for the first control channel in the first CSS and the first USS. If the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for the first control channel in the first CSS and the first USS, and the terminal searches for the second control channel in the second CSS and the second USS. In other words, if the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for a control channel arranged by the base station in the CSSs in the first control channel region and the second control channel region and the USSs in the first control channel region and the second control channel region. Therefore, the number of physical control channels usable within one sub-frame increases. In addition, CSSs where the interference is very small can be utilized.

Accordingly, the monitoring of the second control channel (the monitoring of both the first control channel and the second control channel) is configured through signaling from a base station to a terminal. In this case, the terminal monitors, within one sub-frame, a control channel arranged in the base station at least both in the CSS in the first control channel region and the USS in the second control channel region for which monitoring has been set. This enables, while monitoring a control channel to which control information to be read by a plurality of terminals, such as system information to be arranged in the CSS and information concerning paging, is allocated, the use of an efficient second control channel.

According to a different point of view, furthermore, the physical control channel that uses the same transmission port as that for a cell-specific reference signal, and the physical control channel that uses the same transmission port as that for a UE-specific reference signal are switched over. This allows the base station 101 to perform a demodulation process on the control channel for the terminal 102 using any of a cell-specific reference signal and a UE-specific reference signal in accordance with the situation, thereby implementing resource allocation scheduling taking the performance obtained from the respective reference signals into account. Specifically, in cases such as when the initial access is made by a terminal and when the number of terminals accommodated by a base station is small and there is no shortage of the capacity of a PDCCH, a demodulation process is performed using a cell-specific reference signal by using the control channel to be mapped to the first control channel region. In cases such as when the number of terminals accommodated by a base station increases and there is a shortage of the capacity of a PDCCH, a demodulation process is performed using a UE-specific reference signal by using the control channel mapped to the second control channel region.

In particular, a terminal that performs CoMP communication switches the control channel to a physical control channel that uses the same transmission port as that for a UE-specific reference signal. Since a control channel that is demodulated using a UE-specific reference signal can be subjected to a precoding process suitable for the terminal 102, the terminal 102 can receive a control channel via a transmission channel excellent in transmission performance. In addition, for a control channel notified using a UE-specific reference signal, even in a case where the same control channel is transmitted from a plurality of base stations, a combined channel state obtained from the respective base stations can be estimated. Thus, the terminal 102 does not need to individually estimate the respective channel states of the base stations in CoMP communication, and can achieve an efficient receiving process. In addition, for a control channel that is demodulated using a cell-specific reference signal, there is no need to use any new resource, unlike a UE-specific reference signal, when the terminal 102 demodulates the control channel. Therefore, the base station 101 can efficiently notify the terminal 102 of a control channel without increasing overhead.

To achieve the above advantages, there is no need for a limitation in which the resource (first control channel region) where the first control channel is to be arranged and the resource (second control channel region) where the second control channel is to be arranged are different from each other. Even in a case where the first control channel and the second control channel are arranged in the same resource, such as when both the first control channel and the second control channel use the PDSCH region, the above advantages are achieved as long as transmission ports associated with the respective control channels are those for a cell-specific reference signal and a UE-specific reference signal.

To achieve the above advantages, furthermore, there is no need for a limitation in which the second control channel is a UE-specific physical control channel. Even in a case where the second control channel is a cell-specific physical control channel, such as when the second control channel configuration information is broadcasted via a broadcast channel and is shared and used by a plurality of terminals accommodated by a base station, the above advantages are achieved as long as transmission ports associated with the respective control channels are those for a cell-specific reference signal and a UE-specific reference signal. In this case, the UE-specific reference signal is a pre-configured UE-specific reference signal.

According to a different point of view, furthermore, a physical control channel which may be arranged over an entire CC region in OFDM symbol(s) located in a front part of a physical sub-frame, and a physical control channel which may be arranged over part of a band in OFDM symbols located after this control channel are switched over. This allows the base station 101 to perform a resource allocation and/or scheduling process on the control channel for the terminal 102 while taking the performance of the respective resources into account. That is, in cases such as when the initial access is made by a terminal and when the number of terminals accommodated by a base station is small and there is no shortage of the capacity of a PDCCH or when discontinuous reception is ongoing, a physical control channel which may be arranged over an entire CC region in OFDM symbol(s) located in a front part of a physical sub-frame is used, thereby making it possible to reduce receive power while randomizing inter-cell interference. In cases such as when the number of terminals accommodated by a base station increases and there is a shortage of the capacity of a PDCCH or when inter-cell interference is serious, a physical control channel which may be arranged in part of a band over OFDM symbols located after the above control channel is used to prevent occurrence of interference.

To achieve the above advantages, there is no need for a limitation in which the first control channel is set to a physical control channel that uses the same transmission port as that for a cell-specific reference signal and the second control channel is set to a physical control channel that uses the same transmission port as that for a UE-specific reference signal. Even in a case where the first control channel and the second control channel use the same transmission port, such as when both the first control channel and the second control channel use the same transmission port as that for a cell-specific reference signal, the above advantages are achieved as long as the respective control channels are arranged in different resources in the manner described above. To achieve this advantage, furthermore, there is no need for a limitation in which the second control channel is a UE-specific physical control channel. Even in a case where the second control channel is a cell-specific physical control channel, such as when the second control channel configuration information is broadcasted via a broadcast channel and is shared and used by a plurality of terminals accommodated by a base station, the above advantages are achieved as long as the respective control channels are arranged in different resources in the manner described above.

According to a different point of view, furthermore, a cell-specific physical control channel and a UE-specific physical control channel are switched over. This allows the base station 101 to perform a resource allocation scheduling process on the control channel for the terminal 102 while taking the state of each terminal (such as idle state or connected state) into account. Specifically, in a case such as when the initial access is made by a terminal, when the number of terminals accommodated by a base station is small and there is no shortage of the capacity of a PDCCH, or when discontinuous reception is ongoing, a cell-specific physical control channel that can also be received by a terminal in idle state is used. In a case where the number of terminals accommodated by a base station increases and there is a shortage of the capacity of a PDCCH, the proportion of the use of a UE-specific physical control channel that can be acquired only by a terminal in connected state is increased.

Particularly in a case where there is a serious shortage of the capacity of a control channel, switching to a UE-specific control channel is made. Since a UE-specific control channel can be configured specific to each terminal, the base station 101 can adaptively control the capacity of the total control channel also including a cell-specific control channel. For example, the base station 101 adaptively configures/reconfigures/releases a UE-specific control channel in accordance with the number of terminals that are notified of the control channel. Furthermore, since a UE-specific control channel can be configured individually for each terminal, the base station 101 can implement the configuration/reconfiguration/release of the UE-specific control channel by notifying only the target terminal. In addition, the notification of the control channel via a cell-specific control channel can be implemented without configuring a UE-specific control channel, and therefore the overhead of the control information in RRC signaling can be reduced.

To achieve this advantage, there is no need for a limitation in which the first control channel is set to a physical control channel that uses the same transmission port as a cell-specific reference signal and the second control channel is set to a physical control channel that uses the same transmission port as a UE-specific reference signal. Even in a case where the first control channel and the second control channel use the same transmission port, such as when both the first control channel and the second control channel use the same transmission port as a cell-specific reference signal, the above advantages are achieved as long as the respective control channels are a cell-specific physical control channel and a UE-specific physical control channel. To achieve this advantage, furthermore, there is no need for a limitation in which the resource in which the first control channel is to be arranged and the resource in which the second control channel is to be arranged are different from each other. Even in a case where the first control channel and the second control channel are arranged in the same resource, such as when both the first control channel and the second control channel use the PDSCH region and the first control channel is configured via a broadcast channel, the above advantages are achieved as long as the respective control channels are a cell-specific physical control channel and a UE-specific physical control channel.

In the foregoing description, the description has been given of, but not limited to, the case where control information with which the base station 101 configures the second control channel for the terminal 102 is information specific to the terminal 102. Control information with which the base station 101 configures the second control channel for the terminal 102 may be information specific to the base station 101. For example, the base station 101 may broadcast control information for setting the second control channel to each terminal via a broadcast control channel (BCH; Broadcast Channel). Accordingly, the base station 101 can reduce the overhead of control information for setting the second control channel. In addition, the base station 101 may notify each terminal of control information for setting the second control channel, via RRC signaling, as part of cell-specific configuration information for radio resources (RadioResourceConfigCommon), which is information specific to the base station 101. Accordingly, the base station 101 does not need to schedule the setting of the second control channel for each terminal, thereby reducing the load for scheduling processing.

In the foregoing description, the description has been given of, but not limited to, a method for switching the control channel to be subjected to blind decoding by the terminal 102 in accordance with whether the base station 101 is to configure the UE-specific configuration information on the second control channel for the terminal 102. For example, the base station 101 may notify the terminal 102 of information indicating whether the second control channel is to be configured, via RRC signaling or PDCCH. In this case, control information for setting the second control channel may be broadcasted via a broadcast control channel. In addition, the base station 101 may notify each terminal of control information for setting the second control channel, via RRC signaling, as part of cell-specific configuration information for radio resources, which is information specific to the base station 101. Accordingly, the base station 101 can perform efficient setting of the terminal 102.

In the foregoing description, the description has been given of, but not limited to, a method for switching the control channel to be subjected to blind decoding by the terminal 102 in accordance with whether the base station 101 is to configure the UE-specific configuration information on the second control channel for the terminal 102. For example, the base station 101 may configure, for each sub-frame, whether the second control channel is to be configured, and may notify the terminal 102 of bitmap information indicating the setting via RRC signaling or PDCCH. In this case, control information for setting the second control channel may be broadcasted via a broadcast control channel. In addition, the base station 101 may notify each terminal of control information for setting the second control channel, via RRC signaling, as part of cell-specific configuration information for radio resources, which is information specific to the base station 101. Accordingly, the base station 101 can perform efficient setting of the terminal 102. The base station 101 can also perform efficient scheduling for the terminal 102.

Furthermore, in a case where the base station 101 configures for each sub-frame, using bitmap information, whether the second control channel is to be configured, the setting for each sub-frame may be determined based on the setting of ABS (Almost Blank Subframe) in the base station 101 or any other base station. For example, in a sub-frame for the base station 101 that is transmitted simultaneously with a sub-frame that has been configured to an ABS by a base station neighboring the base station 101, the base station 101 configures the first control channel for the terminal 102. In a sub-frame for the base station 101 that is transmitted simultaneously with a sub-frame that is not configured to an ABS by a base station neighboring the base station 101, the base station 101 configures the second control channel for the terminal 102. An ABS is a sub-frame that is transmitted with reduced transmit power (including no transmission) with respect to channels including the shared channel, the first control channel, and the second control channel. The use of an ABS can reduce interference with a terminal that performs data communication with a base station neighboring a base station that has configured an ABS. Therefore, interference coordination between base stations (ICIC; Inter-cell interference coordination) can be achieved.

Furthermore, the terminal 102 may select a control channel to be subjected to blind decoding, in accordance with a measurement-resource-restriction pattern of which the base station 101 notifies the terminal 102. For example, the measurement-resource-restriction pattern represents information on restricted measurement of RRM (Radio Resource Management)/RLM (Radio Link Control) in a serving cell, information on restricted measurement of RRM in neighboring base stations, and information on restricted measurement of CSI (Channel State Information) in a serving cell. In particular, the information on restricted measurement of CSI (Channel State Information) in a serving cell is configured as two sub-frame subsets for the terminal 102. The terminal 102 measures the channel state in the sub-frames identified by the sub-frame subsets, and performs periodic or aperiodic reporting. The terminal 102 may further select a control channel to be subjected to blind decoding, in accordance with the sub-frame subsets. For example, the terminal 102 performs blind decoding of the first control channel in the sub-frames identified by one of the sub-frame subsets. The terminal 102 also performs blind decoding of the second control channel in the sub-frames identified by the other sub-frame subset. Links between the two sub-frame subsets and the two control channels may be defined in advance, or may be configured by RRC signaling. In a case where the second control channel has not yet been configured by the base station 101 in a sub-frame indicating the blind decoding of the second control channel, the terminal 102 performs blind decoding of the first control channel.

Second Embodiment

A second embodiment of the present invention will be described hereinafter. Similarly to the communication system according to the first embodiment, a communication system according to the second embodiment includes a base station 101 and a terminal 102. The following description will focus on a portion different from the first embodiment of the present invention.

In the communication system according to the second embodiment, the base station 101 has a plurality of cells, and is capable of setting a serving cell in which the base station 101 performs data communication with the terminal 102 through carrier aggregation, where each cell is also called a component carrier (CC) and is capable of setting a specific cell ID.

FIG. 10 is a diagram illustrating an example of a frequency arrangement for cells with carrier aggregation according to the second embodiment of the present invention. In the case illustrated in FIG. 10, the base station 101 can perform carrier aggregation using three cells (CCs, component carriers). In this case, cells with carrier aggregation can be arranged contiguously and/or non-contiguously in the frequency direction, and the respective system bandwidths of the cells can be set different.

In addition, in a case where the base station 101 is to configure carrier aggregation for the terminal 102, the base station 101 can configure a serving cell (serving CC) to be specific to the terminal 102. In this case, the base station 101 can configure one primary cell (PCC, primary CC, PCell) and one or more secondary cells (SCCs, secondary CCs, SCells) for the terminal 102 as serving cells.

The primary cell is a cell in which the terminal 102 performs data communication at a primary frequency, and implements an initial setup process or a re-connection process with the base station 101. The primary cell is a cell identified as a primary cell in a handover process from other cells (base stations, CCs). The primary cell can be changed through the handover process. The primary cell is used for the transmission of an uplink control channel (PUCCH).

The secondary cell is a cell in which the terminal 102 performs data communication at a secondary frequency, and can be configured (also including addition/release/change) via RRC signaling. For example, the secondary cell can be configured at the time when RRC connection is established, and can be used to provide additional radio resources.

In addition, the secondary cell configured using RRC signaling can be set to activation (valid) or deactivation (invalid). Activation or deactivation in the secondary cell is configured to reduce battery consumption in the terminal 102. The terminal 102 does not receive (does not monitor) some or all channels in the secondary cell which is set to deactivation. The setting of activation or deactivation in the secondary cell is performed with a timer for the signaling of the MAC (Media Access Control) layer (MAC signaling) and deactivation. That is, the terminal 102 is notified of bitmap information indicating activation or deactivation for each secondary cell via MAC signaling. If the secondary cell is set to activation, the terminal 102 activates the secondary cell. In a case where the terminal 102 does not receive any control channel and/or data channel by the time designated by the timer for deactivation after the secondary cell is set to deactivation, the secondary cell is deactivated. In a case where the terminal 102 receives a control channel and/or a data channel by the time designated by the timer for deactivation after the secondary cell is deactivated, the secondary cell is activated.

For example, in the example in FIG. 10, the base station 101 configures cell 2 as the primary cell and cell 3 as the secondary cell for the terminal 102. The base station 101 is also capable of setting a second control channel on a cell-by-cell basis for the terminal 102.

In a case where a second control channel is configured for the primary cell by the base station 101, the terminal 102 monitors the second control channel in the primary cell. In a case where no second control channel is configured for the primary cell by the base station 101, the terminal 102 monitors the first control channel in the primary cell. In a case where the secondary cell is activated and a second control channel is configured for the secondary cell by the base station 101, the terminal 102 monitors the second control channel in the secondary cell. In a case where the secondary cell is activated and no second control channel is configured for the secondary cell by the base station 101, the terminal 102 monitors the first control channel in the secondary cell.

Furthermore, in a case where the secondary cell is set to deactivation, the terminal 102 performs a process based on a predefined method or the method of which the terminal 102 is notified by the base station 101. For example, in a case where the secondary cell is set to deactivation, the terminal 102 does not monitor the first control channel and the second control channel in the secondary cell regardless of whether a second control channel has been configured. Thus, the terminal 102 can reduce monitoring processing for control channels. In another example, in a case where the secondary cell is set to deactivation, the terminal 102 does not monitor the second control channel in the secondary cell regardless of whether a second control channel has been configured. Thus, the base station 101 can notify the terminal 102 of control information through the secondary cell. In another example, furthermore, in a case where the secondary cell is set to deactivation and a second control channel has been configured, the terminal 102 monitors the second control channel in the secondary cell. Thus, the base station 101 can notify the terminal 102 of control information through the secondary cell.

Switching as to whether the terminal 102 is to monitor the first control channel or is to monitor the second control channel in each cell may be made using the method described above in the first embodiment. That is, as described in the first embodiment, the switching of control channels at the terminal 102 is controlled in accordance with an explicit or implicit notification from the base station 101 to the terminal 102 as to whether the second control channel is used.

FIG. 11 is a diagram illustrating another example of the UE-specific configuration information for radio resources. In the case illustrated in FIG. 11, the base station 101 configures carrier aggregation in the terminal 102, and configures a second control channel individually for the primary cell and the secondary cell in the terminal 102. In the example illustrated in FIG. 11, the UE-specific configuration information for radio resources includes, in addition to the UE-specific configuration information for the physical channel described in the first embodiment, UE-specific configuration information for the physical channel in the secondary cell (PhysicalConfigDedicatedSCell-r11). The UE-specific configuration information for radio resources in FIG. 11 can be the UE-specific configuration information for radio resources in the primary cell.

The UE-specific configuration information for the physical channel is control information for defining UE-specific settings for the physical channel in the primary cell. The UE-specific configuration information for the physical channel in the secondary cell is control information for defining UE-specific settings for the physical channel in the secondary cell. The UE-specific configuration information for the physical channel and the UE-specific configuration information for the physical channel in the secondary cell each include UE-specific configuration information on the second control channel.

The UE-specific configuration information on the second control channel in FIG. 11 is similar to the UE-specific configuration information on the second control channel described with reference to FIG. 8, and can be independently configured for the primary cell and the secondary cell. The second control channel can be configured for either the primary cell or the secondary cell. In this case, the UE-specific configuration information on the second control channel is configured for only the cell in which the second control channel is configured.

With the use of the method illustrated in FIG. 11, the base station 101 can efficiently set a second control channel in the primary cell and the secondary cell for the terminal 102. In addition, since the UE-specific configuration information for the physical channel in each cell, which includes the UE-specific configuration information on the second control channel, is included in the UE-specific configuration information for radio resources, the base station 101 can perform adaptive setting for the terminal 102 in accordance with the channel state of the downlink or the state of the base station 101.

FIG. 12 is a diagram illustrating another example of the UE-specific configuration information for radio resources. In the case illustrated in FIG. 12, the base station 101 configures carrier aggregation in the terminal 102, and configures a second control channel individually for the primary cell and the secondary cell in the terminal 102. In the example illustrated in FIG. 12, the UE-specific configuration information for radio resources includes UE-specific configuration information for the physical channel. The UE-specific configuration information for the physical channel includes, in addition to the UE-specific configuration information on the second control channel described in the first embodiment, UE-specific configuration information on the second control channel in the secondary cell (XPDCCH-ConfigDedicatedSCell-r11). The UE-specific configuration information on the second control channel in FIG. 12 can be the UE-specific configuration information on the second control channel in the primary cell.

The UE-specific configuration information on the second control channel is used to define UE-specific configuration information on the second control channel in the primary cell. The UE-specific configuration information on the second control channel in the secondary cell is used to define UE-specific configuration information on the second control channel in the secondary cell. The UE-specific configuration information on the second control channel and the UE-specific configuration information on the second control channel in the secondary cell each include sub-frame configuration information on the second control channel.

The sub-frame configuration information on the second control channel in FIG. 12 is similar to the sub-frame configuration information on the second control channel described in FIG. 8, and can be independently configured for the primary cell and the secondary cell. The second control channel can be configured for either the primary cell or the secondary cell. In this case, the UE-specific configuration information on the second control channel or the sub-frame configuration information on the second control channel is configured for only the cell in which the second control channel is configured.

With the use of the method illustrated in FIG. 12, the base station 101 can efficiently set a second control channel in the primary cell and the secondary cell for the terminal 102. In addition, since the UE-specific configuration information on the second control channel in each cell is included in the UE-specific configuration information for the physical channel, the base station 101 can perform adaptive setting for the terminal 102 in accordance with the channel state of the downlink or the state of the base station 101 while reducing the overhead of the control information for the terminal 102.

FIG. 13 is a diagram illustrating another example of the UE-specific configuration information on the second control channel. In the following description, the description will be given of a difference from the UE-specific configuration information on the second control channel described in FIG. 8. The sub-frame configuration information on the second control channel included in the UE-specific configuration information on the second control channel further includes an assigned cell ID (schedulingCellId-r11).

The assigned cell ID is information indicating a cell in which the second control channel is configured with the sub-frame configuration information on the second control channel. The assigned cell ID is selected from among the serving cell set in the terminal 102 and/or a cell that can be configured as carrier aggregation by the base station 101. For the assigned cell ID, one or a plurality of cells can be configured. In a case where a plurality of assigned cell IDs are configured, part or all of the sub-frame configuration information on the second control channel can be configuration information common to the respective cells. In a case where a plurality of assigned cell IDs are set, furthermore, the sub-frame configuration information on the second control channel can be configured individually the respective cells.

Furthermore, in a case where the cell identified by the assigned cell ID indicates a secondary cell and this secondary cell is set to deactivation, the terminal 102 performs a process based on a predefined method or the method notified by the base station 101. For example, the terminal 102 does not monitor the first control channel and the second control channel in the secondary cell regardless of the notified sub-frame configuration information on the second control channel. Thus, the terminal 102 can reduce monitoring processing for control channels. In another example, the terminal 102 monitors either the first control channel or the second control channel in the secondary cell regardless of the setting of deactivation for the secondary cell. Thus, the base station 101 can notify the terminal 102 of control information through the secondary cell.

The UE-specific configuration information on the second control channel illustrated in FIG. 13 can be notified and configured as control information for the primary cell and/or the secondary cell. In a case where the UE-specific configuration information on the second control channel illustrated in FIG. 13 is notified from only one cell, the UE-specific configuration information on the second control channel is preferably notified and configured as control information for the primary cell.

With the use of the method illustrated in FIG. 13, the base station 101 can efficiently configured a second control channel in the primary cell and the secondary cell for the terminal 102. In addition, since the sub-frame configuration information on the second control channel is configured so as to include the assigned cell ID, the base station 101 can perform adaptive setting for the terminal 102 in accordance with the channel state of the downlink or the state of the base station 101 while reducing the overhead of the control information for the terminal 102.

With the use of the methods described above, in addition to the advantages described in the first embodiment described above, the base station 101 can efficiently configure carrier aggregation for the terminal 102, and can efficiently configure a second control channel in the primary cell and/or the secondary cell.

Next, a description will be given of a SS in the PCell and/or the SCell and an example of search therein. A CSS and a USS are set for the PCell. On the other hand, only a USS is set for the SCell. Conversely, the PCell can be said to be a cell having a CSS, and the SCell can be said to be a cell having only a USS (having no CSS).

The monitoring of the second control channel is configured in any of the cells through signaling from a base station to a terminal. In this case, the terminal monitors both the first control channel in at least the PCell within one sub-frame and the second control channel for which monitoring has been configured. That is, the terminal searches for the first control channel in the SS in which the first control channel is searched for within one sub-frame, and searches for the second control channel in the SS in which the second control channel is searched for.

Any of the following methods may be used as an example of the SS setting method or the monitoring method in a case where the monitoring of the second control channel has been set in the PCell.

(1) A CSS and a USS (first USS) are set in the first control channel region in the PCell, and a USS (second USS) is set in the second control channel region in the PCell. A USS (third USS) is set in the first control channel region in the SCell. If the monitoring of the second control channel is not configured, a terminal searches for the first control channel in the CSS and the first USS, and searches for the first control channel in the third USS. If the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for the first control channel in the CSS, searches for the first control channel in the third USS, and searches for the second control channel in the second USS. In other words, if the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for a control channel arranged by the base station in the CSS in the first control channel region in the PCell, the USS in the second control channel region in the PCell, and the USS in the first control channel region in the SCell. Therefore, the number of physical control channels usable within one sub-frame increases. In addition, the increase in the number of blind decoding attempts can be suppressed.

(2) A CSS and a USS (first USS) are set in the first control channel region in the PCell, and a USS (second USS) is set in the second control channel region in the PCell. A USS (third USS) is set in the first control channel region in the SCell. If the monitoring of the second control channel is not configured, a terminal searches for the first control channel in the CSS and the first USS, and searches for the first control channel in the third USS. If the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for the first control channel in the CSS and the first USS, searches for the first control channel in the third USS, and searches for the second control channel in the second USS. In other words, if the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for a control channel arranged by the base station in the CSS in the first control channel region in the PCell, the USSs in the first control channel region and the second control channel region in the PCell, and the USS in the first control channel region in the SCell. Therefore, the number of physical control channels usable within one sub-frame increases. In addition, the CSS can be shared by a plurality of terminals within a cell.

(3) A CSS (first CSS) and a USS (first USS) are set in the first control channel region in the PCell, and a CSS (second CSS) and a USS (second USS) are set in the second control channel region in the PCell. A USS (third USS) is set in the first control channel region in the SCell. If the monitoring of the second control channel is not configured, a terminal searches for the first control channel in the first CSS and the first USS, and searches for the first control channel in the third USS. If the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for the first control channel in the first CSS and the first USS, searches for the first control channel in the third USS, and searches for the second control channel in the second CSS and the second USS. In other words, if the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for a control channel arranged by the base station in the CSSs in the first control channel region and the second control channel region in the PCell, the USSs in the first control channel region and the second control channel region in the PCell, and the USS in the first control channel region in the SCell. Therefore, the number of physical control channels usable within one sub-frame increases. In addition, CSSs where the interference is very small can be utilized.

Next, any of the following methods may be used as an example of the SS setting method or the monitoring method in a case where the monitoring of the second control channel has been configured in the SCell.

(4) A CSS and a USS (first USS) are set in the first control channel region in the PCell. A USS (third USS) is set in the first control channel region in the SCell, and a USS (fourth USS) is set in the second control channel region in the SCell. If the monitoring of the second control channel is not configured, a terminal searches for the first control channel in the CSS and the first USS, and searches for the first control channel in the third USS. If the monitoring of the second control channel is set through signaling from a base station to a terminal, the terminal searches for the first control channel in the CSS and the first USS, and searches for the second control channel in the fourth USS. In other words, if the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for a control channel arranged by the base station in the CSS and USS in the first control channel region in the PCell and the USS in the second control channel region in the SCell. Therefore, the number of physical control channels usable within one sub-frame increases. In addition, the increase in the number of blind decoding attempts can be suppressed.

(5) A CSS and a USS (first USS) are set in the first control channel region in the PCell. A USS (third USS) is set in the first control channel region in the SCell, and a USS (fourth USS) is set in the second control channel region in the SCell. If the monitoring of the second control channel is not configured, a terminal searches for the first control channel in the CSS and the first USS, and searches for the first control channel in the third USS. If the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for the first control channel in the CSS and the first USS, searches for the first control channel in the third USS, and searches for the second control channel in the fourth USS. In other words, if the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for a control channel arranged by the base station in the CSS and USS in the first control channel region in the PCell, the USS in the first control channel region in the SCell, and the USS in the second control channel region in the SCell. Therefore, the number of physical control channels usable within one sub-frame increases. In addition, the CSS can be shared by a plurality of terminals within a cell.

Next, any of the following methods may be used as an example of the SS setting method or the monitoring method in a case where the monitoring of the second control channel has been configured in both the PCell and the SCell.

(6) A CSS and a USS (first USS) are set in the first control channel region in the PCell, and a USS (second USS) is set in the second control channel region in the PCell. A USS (third USS) is set in the first control channel region in the SCell, and a USS (fourth USS) is set in the second control channel region in the SCell. If the monitoring of the second control channel is not configured, a terminal searches for the first control channel in the CSS and the first USS, and searches for the first control channel in the third USS. If the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for the first control channel in the CSS, and searches for the second control channel in the second USS and the fourth USS. In other words, if the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for a control channel arranged by the base station in the CSS in the first control channel region in the PCell, the USS in the second control channel region in the PCell, and the USS in the second control channel region in the SCell. Therefore, the number of physical control channels usable within one sub-frame increases. In addition, the increase in the number of blind decoding attempts can be suppressed.

(7) A CSS and a USS (first USS) are set in the first control channel region in the PCell, and a USS (second USS) is set in the second control channel region in the PCell. A USS (third USS) is set in the first control channel region in the SCell, and a USS (fourth USS) is set in the second control channel region in the SCell. If the monitoring of the second control channel is not configured, a terminal searches for the first control channel in the CSS and the first USS, and searches for the first control channel in the third USS. If the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for the first control channel in the CSS and the first USS, searches for the second control channel in the second USS, searches for the first control channel in the third USS, and searches for the second control channel in the fourth USS. In other words, if the monitoring of the second control channel is configured through signaling from a base station to a terminal, the terminal searches for a control channel arranged by the base station in the CSS and USS in the first control channel region in the PCell, the USS in the first control channel region in the SCell, the USS in the second control channel region in the PCell, and the USS in the second control channel region in the SCell. Therefore, the number of physical control channels usable within one sub-frame increases. In addition, the CSS can be shared by a plurality of terminals within a cell.

As described above, the monitoring of the second control channel in any of the cells is configured through signaling from a base station to a terminal. In this case, the terminal monitors, within one sub-frame, a control channel to be arranged in the base station in both at least the CSS in the first control channel region in the PCell and the USS in the second control channel region for which monitoring has been configured. This enables, while monitoring a control channel to which control information to be read by a plurality of terminals, such as system information to be arranged in the CSS or information concerning paging, is allocated, the use of an efficient second control channel.

In the foregoing description, the description has been given of, but not limited to, the case where control information for each cell is set in order to configure a second control channel for the primary cell and the secondary cell. That is, in a case where the control information described in the first embodiment described above is set for control information for the primary cell and a second control channel is configured for the secondary cell, the control information for the secondary cell described in the first embodiment described above may further be set. In the above example, furthermore, the description has focused on the case where one SCell is used. Similar application can be made in the case where there are a plurality of SCells.

Alternatively, in the foregoing description, the description has been given of, but not limited to, the case where it is possible to configure a second control channel for both the PCell and the SCell. For example, it may be possible to configure a second control channel region for only the PCell. In this case, the setting of the second control channel region and the setting of monitoring indicate the setting of a second control channel in the PCell. Therefore, complexity of signaling can be reduced.

In the foregoing embodiments, by way of example, but not limited thereto, a resource element and/or a resource block is used as the unit of mapping a data channel, a control channel, a PDSCH, a PDCCH, and a reference signal, and a sub-frame and/or a radio frame is used as the unit of transmission in the time direction. Similar advantages can be achieved with the use of the domain composed of any desired frequency and time and the time unit instead of them. In the foregoing embodiments, the description has been given of, but not limited to, the case where demodulation is carried out using an RS subjected to a precoding process, and a port equivalent to the layer of MIMO is used as the port corresponding to the RS subjected to the precoding process. Similar advantages can be achieved by applying the present invention to ports corresponding to different reference signals. For example, in place of a Precoded RS, an Unprecoded RS may be used, and a port that is equivalent to a port or physical antenna (or a combination of physical antennas) that is equivalent to the output edge after the precoding process has been performed may be used as a port.

A program operating in the base station 101 and the terminal 102 according to the present invention is a program (a program for causing a computer to function) for controlling a CPU and so forth so as to implement the functions of the foregoing embodiments according to the present invention. Such information as handled by devices is temporarily accumulated in a RAM while processed, and is then stored in various ROMs and HDDs. The information is read by the CPU, if necessary, for modification/writing. A recording medium having the program stored therein may be any of semiconductor media (for example, a ROM, a non-volatile memory card, etc.), optical recording media (for example, a DVD, an MO, an MD, a CD, a BD, etc.), magnetic recording media (for example, a magnetic tape, a flexible disk, etc.), and so forth. Furthermore, in addition to the implementation of the functions of the embodiments described above by executing the loaded program, the functions of the present invention may be implemented by processing the program in cooperation with an operating system, any other application program, or the like in accordance with instructions of the program.

In a case where the program is distributed to be usable on market, the program may be stored in a transportable recording medium for distribution, or may be transferred to a server computer connected via a network such as the Internet. In this case, a storage device in the server computer also falls within the scope of the present invention. In addition, part or the entirety of the base station 101 and the terminal 102 in the embodiments described above may be implemented as an LSI, which is typically an integrated circuit. The respective functional blocks of the base station 101 and the terminal 102 may be individually built into chips, or some or all of them may be integrated and built into a chip. The method for forming an integrated circuit is not limited to LSI, and may be implemented by a dedicated circuit or a general-purpose processor. In the case of the advent of integrated circuit technology replacing LSI due to the advancement of semiconductor technology, it is also possible to use an integrated circuit based on this technology.

While embodiments of this invention have been described in detail with reference to the drawings, a specific configuration is not limited to these embodiments, and the invention also includes design changes and the like without departing from the essence of this invention. In addition, a variety of changes can be made to the present invention within the scope defined by the claims, and embodiments that are achieved by appropriately combining respective technical means disclosed in different embodiments are also embraced within the technical scope of the present invention. Furthermore, a configuration in which the elements described in the foregoing embodiments and capable of achieving similar advantages are interchanged is also embraced within the technical scope of the present invention.

REFERENCE SIGNS LIST

101, 1401 base station, 102, 1402, 1403, 1504 terminal, 103, 1404, 1405, 1505, 1506 downlink, 201, 305 higher layer, 202 data channel generation unit, 203 UE-specific reference signal multiplexing unit, 204 precoding unit, 205 cell-specific reference signal multiplexing unit, 206 transmit signal generation unit, 207 transmission unit, 301 reception unit, 302 reception signal processing unit, 303 control channel processing unit, 304 data channel processing unit, 1501 macro base station, 1502 RRH, 1503 line. 

1-12. (canceled)
 13. A terminal for performing OFDM (Orthogonal Frequency-Division Multiplexing) communication with a base station configuring a cell, comprising: a control channel processing unit configured to monitor, in a sub-frame, a first control channel in a cell-specific search space that is a search space common within the cell, and a first control channel in a UE-specific search space that is a search space specific to the terminal, wherein in a case where monitoring of a second control channel different from the first control channel that is in the cell-specific search space or the UE-specific search space is configured, the control channel processing unit monitors, in a sub-frame, at least both the first control channel in the cell-specific search space and the second control channel in the UE-specific search space.
 14. The terminal according to claim 13, wherein the first control channel in the cell-specific search space is a control channel which can include system information, paging, and control information associated with random access instruction.
 15. The terminal according to claim 13, wherein the first control channel in the cell-specific search space is transmitted using a transmission port used for transmission of a cell-specific reference signal, and the second control channel in the UE-specific search space is transmitted using a transmission port used for transmission of a UE-specific reference signal.
 16. The terminal according to claim 13, wherein the first control channel in the cell-specific search space is mapped on an OFDM symbol from the top OFDM symbol to an OFDM symbol with a certain number in the sub-frame, and the second control channel in the UE-specific search space is mapped on an OFDM symbol subsequent to an OFDM symbol with a certain number in the sub-frame.
 17. The terminal according to claim 13, wherein the first control channel in the cell-specific search space is a control channel usable while the terminal is in idle state and in connected state, and the second control channel in the UE-specific search space is a control channel usable only while the terminal is in the connected state.
 18. A communication method for a terminal that performs OFDM communication with a base station configuring a cell, comprising: a step of monitoring, in a sub-frame, a first control channel in a cell-specific search space that is a search space common within the cell, and a first control channel in a UE-specific search space that is a search space specific to the terminal; and a step of, in a case where monitoring of a second control channel different from the first control channel that is in the cell-specific search space or the UE-specific search space is configured, monitoring, in a sub-frame, at least both the first control channel in the cell-specific search space and the second control channel in the UE-specific search space.
 19. A non-transitory recoding medium used for a terminal that performs OFDM communication with a base station configuring a cell, the non-transitory recoding medium storing a program implementing: a function of monitoring, in a sub-frame, a first control channel in a cell-specific search space that is a search space common within the cell, and a first control channel in a UE-specific search space that is a search space specific to the terminal; and a function of, in a case where monitoring of a second control channel different from the first control channel that is in the cell-specific search space or the UE-specific search space is configured, monitoring, in a sub-frame, at least both the first control channel in the cell-specific search space and the second control channel in the UE-specific search space. 